1. Field of the Invention
The present invention relates to remediation of contaminated soil, particularly remediation by bacteria, and more particularly the use of microbes for the degradation of chemical pollutants. The United States Government has rights in the present invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of the Background
Use of microbes in industrial processes and pollution control continues to increase. Microbes are efficient and economical transformers of chemical compounds and intense effort is underway to find or engineer microbes suitable for various industrial purposes.
It was thought that certain types of compounds would not lend themselves to microbial degradation, compounds such as xenobiotic or synthetic chemicals, since these chemicals are usually toxic, do not occur naturally in the environment, and become recalcitrant when placed in a natural environment. However, research has shown that these, too, can be degraded. The range of applications for microbial degradation is thus greatly increased. In the treatment of effluents from sewage treatment plants and other industrial processes, microbial transformation of compounds is now common practice.
The major drawbacks in the utilization of microbes in remediation of contaminated groundwater and soils are the small size and sparse distribution of in situ microbial populations. Although found throughout the subsurface even to great depths, the concentration of microbes usually available for remediation of a plume of contaminants is generally low.
Stimulating microbes to degrade the contaminants is a possible solution to the drawbacks of small population numbers and sparse distribution. Generally, nutrients are injected or infused into the soil to provide nutrients that have become limiting to the microbes and thus promote the growth of microbial degraders. Many of these nutrients will stimulate the growth of both degraders and nondegraders. Identifying those microbes most effective at breaking down a given contaminant is a more efficient approach to remediation. Once the correct microbes are identified, nutrients optimal to those microbes can be supplied to enhance population growth. The identifying process should be as simple as possible in order to save time. Conventional methods of bioremediation site characterization, monitoring and control use culturing techniques which require days or weeks to incubate a colony large enough to visualize. Culturing is inaccurate because it is not specific and does not allow all environmentally active microbes to grow.
Microbes move in the ground sometimes randomly, sometimes with the flow of ground water and sometimes in response to taxis effects. A particular type of taxis is chemotaxis, defined as the movement of microorganisms toward or away from a chemical. Chemotaxis is positive or negative depending on whether it is toward or away from the chemical, respectively. Through chemotaxis, microbes seek optimum surroundings, such as those having nutrients, and avoid unfavorable ones, such as those without nutrients or with toxins. The movement is characterized by two actions, one called "runs" and one called "twiddles". When the organism runs, it swims steadily in a gently curved path. When it twiddles, it stops and jiggles in place. After a twiddle, the organism runs off again in a new direction. A twiddle is a random event, and following a twiddle, the direction for the next run is also random. When a chemical gradient is added, the random movements become biased. As the organism experiences higher concentrations of an attractant, the runs become longer and the twiddles become less frequent.
Although chemotaxis is not completely understood, it is believed that a set of proteins, called chemoreceptors, exist in the cell wall or membrane of the microorganisms and are specific for groups of closely-related compounds. A second set of proteins, methyl-accepting chemotaxis proteins (MCP), translates chemotactic signals to the flagellar motor of the microorganism. Methylation catalyzes the MCP to produce a chemical mediator that diffuses the flagellar motor so that it rotates in the same direction, thereby lengthening runs and decreasing twiddles of the microorganism's motion until the microorganism can no longer sense a concentration gradient in the chemical substance. At some point the microorganism's receptors saturate and no further increase in the substance will increase the chemotacic response.
The chemotaxis of a microbe can be used to advantage when determining which microbe will degrade a given pollutant.